The present invention relates to an apparatus for maintaining the orientation of an antenna system with respect to a beacon. The apparatus includes a monopulse cluster for receiving radio signals in four quadrants and converting the signals into sum and difference signals .SIGMA.x, .SIGMA.y and .DELTA.x, .DELTA.y respectively. It further includes transforming means for transforming the sum and difference signals to the intermediate frequency, amplifying and phase detecting said sum and difference signals to derive the error voltages f.DELTA.x, f.DELTA.y from the then obtained signals to align the antenna system with a position determined by the beacon. The sum and difference signals are defined in the conventional way with respect to an x,y coordinate system in a plane perpendicular to the axis between the antenna system and the beacon.
With the direct transmission of television programs via satellites, the antennas mounted on a satellite have to be optimally directed to the target area concerned. As a satellite, even in a geostationary orbit, performs small movements relative to the earth, this target area will have to be tracked, as it were, constantly. For this purpose, a beacon ground station is used, emitting a high-frequency tracking signal, preferably a continuous signal. In such a case, the relevant angle tracking data is obtainable in the satellite by the application of the monopulse principle; the received signals provide the sum and difference signals .SIGMA.x, .SIGMA.y and .DELTA.x, .DELTA.y, respectively.
The beacon may be so arranged that the target area concerned is covered by aligning the antennas with the beacon; in such a case, the .DELTA.x, .DELTA.y difference signals will be zero. Geographic conditions, however, may require to position the beacon away from the central position. In this case, the antennas must be aligned with the beacon at fixed angles with respect to the axis between satellite and beacon; one or both .DELTA.x, .DELTA.y difference signals will then differ from zero. To be able to derive the relevant angular data from the received signals of the beacon in the satellite, in-phase coherent detection should be possible. If the transmitter and the receiver are positioned together, the received signals are detected by utilising, amongst others, a signal aligned in phase with the transmission signal. Such a signal however is not available here, as the beacon and the satellite are far apart.